The present invention relates to grinding mills, and particularly to a centrifugal ring-roller mill incorporating in the mill cylinder a decelerating means, a grinding roller means, a classifying means and a finished product conveying means. A motor drives the said means by a single shaft. This mill is suitable solid ores that are soft or hard viscous, and fibrous raw materials to the desired fineness.
Typical of known roller mills is the Raymond ring-roller mill, which is vertically mounted and contains therein a centrifugal grinding roller device and a fixed separator. The grinding roller device is operated by a drive means mounted outside the mill through a speed reducing means, drive means, and a horizontal drive shaft. A blower, also mounted outside the mill, is used for creating an upward air stream in the mill to sweep finely ground material or particles through the fixed separate vanes of the separator into a discharge pipe. Such a mill is rather complicated in construction and difficult to maintain and repair. It also occupies a large space, and requires high power. In operation the mill presents further problems such as vibration, noise, and dust clouds. Moreover, as far as grinding efficiency is concerned, particles finer than 300 U.S. standard mesh can hardly be produced, and grinding fibrous and viscous raw materials can be difficult.
In order to overcome the above disadvantages, the inventor provided an improved roller mill, that is described in U.S. Pat. No. 3,955,766. Mounted in ascending order in this roller mill are a speed reducer device, a centrifugal grinding roller device, a classifier device and dust collector device. All the devices are driven by a single vertical shaft that is driven by an electric motor mounted on top of the mill so as to obviate the need for a drive means, driven means and blower located outside the mill. This not only renders the mill simple in construction and compact in size, but can also increase grinding efficiency. Furthermore, the roller mill is suitable for grinding various raw materials including solid, fibrous, and viscous materials, with the finished particles being finer than 1,000 mesh.
While that roller mill enjoys several advantages over the previously described ring-roller mill, the inventor has performed subsequent experiments and has found that although there is no denying the fact that using centrifugal force resulting from rotating rollers as the grinding force yields positive grinding effects, the centrifugal force varies in accordance with the rotational speed and weight of the rollers and thus may result in different grinding forces. Under normal low-speed operating conditions, the rollers will not cause any problem. However, if the rotational speed of the rollers is increased in order to facilitate grinding of the raw materials or to obtain greater capacity or output, an excessively high grinding pressure results from the increased centrifugal force due to the increase in rotational speed of the rollers. The excessively high grinding pressure tends to compress part of the ground fine particles into flakes which may adhere to the grinding ring and the peripheral wall of the rollers or drop down on the bottom of the mill instead of being carried in an air stream to the classifier. Also, the excessively high grinding pressure often gives rise to great vibration, noise, and wear of the rollers and the grinding ring by subjecting both to crushing and friction. In addition, since in there is no clearance between the centrifugal rollers and the grinding ring and relatively high power is required to start operation due to the great friction and vibration at the time of actuation. Especially, in case of an unexpected loss of power during operation, it will be extremely difficult to re-start the rollers after the power supply resumes unless the raw material already fed into the mill has been cleared away, since the internal working parts of the mill are fully loaded. In addition, since the aforesaid rollers are each held in suspension by a support arm, when the rollers need repairing or replacing due to wear, the support arm and the shaft of the associated roller must be disassembled as a whole so that roller can be removed, thus inconveniencing maintenance and repair work. Further, the support arm occupies so large a space that not only is the effective working space in the separation chamber diminished, but the support arm, when rotating, also hinders the formation of an upwardly spiralling flow of ground particles and air below it to such an extent that the particles come together and cause the grinder to become plugged.